The long-term aim of this work is to identify vaccine candidates to prevent severe malaria syndromes through understanding natural protective immune response. Plasmodium falciparum parasite-infected erythrocytes (IE) adhere to the host endothelium and to red blood cells. The adhesion is mediated by a large family (~60 members) of parasite multidomain variant proteins called PfEMP1. They express on the surface of IE in a mutually-exclusive manner. IE adhesion may result in severe complications including cerebral malaria, pregnancy malaria, severe anemia and respiratory distress, major contributors to malaria mortality. Severe malaria episodes are rare and almost never occur more than 1 or 2 times in a lifetime. This epidemiology supports our first hypothesis that only a limited number of parasite lines, possibly expressing particular variants of PfEMP1 that determine specific parasite adhesion, may be the source of severe disease. Immunological profiling of sera for reactivity against different antigens is an important method for assessing acquired immunity and identifying potential vaccine candidates. However, relating immune responses to malaria resistance is not straightforward since exposed individuals are typically infected repeatedly throughout life, and develop diversified immune responses against multiple antigens, in many cases without comprehensible relevance to disease severity. Our second hypothesis is that functional antibody responses that inhibit adhesion of IE to host receptors are likely to have a stronger association to protection from severe malaria. Extensive studies in pregnancy malaria strongly support both our hypotheses. During our previous work we collected thousands of serum samples obtained in longitudinal cohort studies of children living in malaria endemic areas of Africa. We also constructed functional genome-wide PfEMP1 domain arrays and used them to identify domains that bind specifically to the two main adhesion host receptors in malaria (ICAM1 and CD36). We further demonstrated that total anti-domain IgG and functional antibodies (that block adhesion of PfEMP1 domain to its receptor) can be quantified in the plasma samples using our multiplexed high throughput sample-sparing platform. To better understand the protective humoral immune responses in severe malaria, we propose the following specific aims: 1) Measure serum total IgG reactivity against a genome-wide array of PFEMP1 domains, and measure functional serum activity that inhibits CD36 and ICAM1 receptor binding to relevant PFEMP1 domains. We will test about 5000 children's sera including ~200-400 severe malaria cases. 2) Associate defined antibody responses (total and functional) against PfEMP1 domains with disease parameters and protection against severe disease. We will perform a broad range of statistical analyses using the longitudinal data obtained in Aim 1. This would be the first genome-wide study of seroreactivity and a comprehensive study of functional immune responses against PfEMP1 proteins. This study may provide candidates for development of vaccines against severe malaria. PUBLIC HEALTH RELEVANCE: The long-term aim of this work is to identify vaccine candidates to prevent severe malaria syndromes caused by Plasmodium falciparum. These syndromes kill over 1 million African children each year. Specific adhesion of parasites in vasculature mediates these severe complications. Various host endothelial receptors and domains of parasite PfEMP1 proteins that express on the surface of infected red blood cells are responsible for this adhesion. In most children naturally developed immunity prevents severe disease. Inhibition of adhesion by natural IgG might be one of the protective factors. In this proposal we will measure host immune responses (IgG reactivity and inhibition of adhesion to the two main host receptors) against a genome-wide array of PfEMP1 proteins using 5000 children's sera collected in endemic areas in East Africa. These measured immune responses will be associated with various disease parameters to understand protection and to identify PfEMP1 domains that can be developed further into vaccines directed against severe malaria.